The economic value of roots arise not only from harvested roots, but also from the ability of roots to alter the soil in which they grow and to funnel nutrients to support growth and increase vegetative material, seeds, fruits, etc.
Roots have four main functions. First, they anchor the plant in the soil. Second, they facilitate and regulate the molecular signals and molecular traffic between the plant, soil and soil fauna. Third, the root provides a plant with nutrients gained from the soil or growth medium. Fourth, they condition local soil chemical and physical properties. Roots arise from meristems cells that are protected by a root cap during root elongation, but as the root grows out, the cap cells abscise and the remaining cells differentiate to the tip. Depending on the plant species, some surface cells of roots can develop into root hairs. Some roots persist for the life of the plant, others gradually shorten as the ends slowly die back and some may cease to function altogether due to external influences.
Because plants are sessile organisms, their survival is critically dependent on rapid adaptation to environmental changes. In the soil, change can arise from alteration of the concentration of oxygen or carbon dioxide, nutrient availability, the presence (or absence) of microorganisms and overall soil humidity. For example, oxygen levels in the rhizosphere decrease rapidly during flooding. Hypoxic or anoxic conditions occur in submerged plant tissues and can have lasting effects on the subsequent growth and/or development of the plant.
Roots are also the sites of intense chemical and biological activities and as a result can strongly modify the soil they contact. For example, roots secrete a wide variety of high and low molecular weight molecules into the rhizosphere in response to biotic and abiotic stresses. They are also capable of absorbing toxic substances from the soil and then storing or modifying the toxins, resulting in soil improvement.
Roots coat themselves with surfactants and mucilage to facilitate these types of activities. Specifically, roots attract and interact with beneficial microfauna and flora that help to mitigate the effects of toxic chemicals, pathogens and stress in addition to facilitating water and nutrient assimilation and mobilization. Nutrients can take the form of ions and organic and inorganic compounds. Uptake of nutrients by roots produces a “source-sink” effect in a plant. The greater the source of nutrients, the larger “sinks” (such as stems, leaves, flowers, seeds, fruits, etc.) can grow.
Currently, transient gene expression has been applied to dicot species using the hairy root system to do a quick gene testing in roots, but establishing the hairy root system for maize and delivering transgenes in roots using Agrobacterium rhizogenes has been difficult. Generating transgenic maize plants with a callus tissue system by standard protocols uses a whole cycle of the transformation process which is a time-consuming process.
To date, there is only limited ability to efficiently and quickly test genes and root promoters in vivo in a root, for example, to assess the strength of a promoter in a root, to assess a gene's effect on the tolerance of roots to pests that attack roots (e.g., insects, fungi, bacteria, viruses, or nematodes) or to assess a gene's effect on the nutritional composition of roots for human food or animal feed applications. Thus a need exists for a highly efficient way to test polynucleotides in the root of a plant and generate plants expressing them in the root.